268 



Basic Structure of Vertebrates 



mouth, gill-clefts, and anus, arise by very similar processes. At 

 the appropriate locality, enteric endoderm and superficial ectoderm 

 approach one another and coalesce. The resulting double layer then 

 thins out until perforation occurs. 



The notochord, whose development is initiated by an upward fold- 

 ing of middorsal endoderm (Fig. 213D-F), early becomes detached from 

 the enteric endoderm and acquires its characteristic cylindric form. 

 The enteric endoderm meanwhile closes in beneath the notochord and 

 restores the integrity of the dorsal wall of the enteron (Fig. 213G). As 



,NP ,NC 



Fig. 222. Amphioxiis. Median longitudinal sec- 

 tion of an embryo having two mesodermal pouches, 

 a stage approximately like that of the transverse 

 section in Fig. 213E. The blastopore, roofed over 

 by ectoderm, has become the neurenteric canal. 

 (A) Archenteron; (EC) ectoderm; (EN) endoderm; 

 (NG) endoderm destined to become notochord; 

 (NE) neurenteric canal; (NP) neural plate; (P) 

 neuropore. (X350.) After Hatschek. Courtesy, 

 Neal and Rand: "Chordate Anatomy," Phila- 

 delphia, The Blakiston Company.) 



the embryo increases in length, the notochord grows within itself and 

 receives accessions from the active blastoporal region with which its 

 posterior end remains for some time connected (Fig. 214). 



The more anterior mesodermal pouches (or somites), soon after 

 their formation and long before the more posterior somites have been 

 developed, begin to acquire their characteristic differentiation. The 

 pouch expands, especially ventralwards, and its cavity is correspond- 

 ingly enlarged. That part of its wall lying against the notochord be- 

 comes much thickened, while elsewhere the wall remains relatively 

 thin. The expansion of the pouches continues until the walls of right 

 and left pouches meet in the median plane beneath the enteric endo- 

 derm. At this stage three regions of the mesoderm may be distin- 

 guished: the thickened part lying alongside the notochord, an outer 

 thin layer contiguous to the ectoderm, and an inner thin layer similarly 



